1. Field of the Invention
The present invention is related to electromagnetic waveguides, and particularly to an improved waveguide component comprising a manually adjustable attenuator.
2. Background of the Invention
Waveguides are used to guide and carry waves, such as electromagnetic, light, or sound waves. The type of waveguide used is dependent on the type of wave to be propagated. The most common waveguide design is a simple hollow metal conductor tube inside which travels a wave, eventually exiting and propagating outward and away from the exit point of the tube. Certain types of waveguides pass the wave through a specific medium, such types including air filled waveguides, dielectric filled waveguides, slot-line waveguides, and slot-based waveguides etc. Typical waveguides are made from materials such as brass, copper, silver, aluminum, or any other metal exhibiting low bulk resistivity.
Waveguides are becoming more commonly used in the millimeter wave and sub-millimeter wave industry, which generally includes frequencies above 30 GHz. This high frequency band of electromagnetic waves is more commonly becoming useful for many new products and services, such as high-resolution imaging systems, high-resolution radar systems, point-to-point communications and point-to-multipoint communications.
Reducing the amplitude of a wave is a common and generally simple practice in the waveguide industry. This is most conveniently and effectively accomplished through the use of adjustable attenuators for waveguides that serve to reduce the amplitude of the wave without distorting the waveguide passing therethrough. This is generally accomplished through an actuatable card or fin in insertable relation with the pathway of the waveguide, absorbing the propagation of the wave to a degree desired by the operator. The dissipative effectiveness of such a card or fin is at its greatest when the card or fin is positioned parallel to and at maximum depth within the strongest part of the electric field within the waveguide.
As is typical in industries that frequently require hardware miniaturization, the waveguide industry is encountering challenges associated with precisely machining the smaller and smaller waveguides and attenuators demanded by high frequency applications. Because in general, higher frequency waves require a smaller waveguide (and relatedly, waveguide attenuators construction), the problems associated with tolerances to which parts are machined in lower frequency installations are multiplied in higher frequency installations. That is, state of the art machining tolerance acceptable at low frequencies will become a major concern in parts made for operation at higher frequencies, particularly in the millimeter wave and sub-millimeter wave range. The accuracy at which an attenuator is constructed directly affects the attenuator's electrical performance.
3. Description of the Related Art
Adjustable attenuators were introduced in the 1950s and generally employed the basic resistive fin or card method as described above. In these systems, an opening is provided and the attenuation card is inserted therethrough and into the path of a wave propagating down a waveguide. Frequencies in use during the early phase of the industry were generally lower than those used today, and hence precision machining of parts and the critical placement of the card were not an issue then as it has become today.
As described above, the dissipative effect of the resistive card varies with the positioning of the card within the waveguide. Moving the card into and out of the guide or moving the fin between the maximum field region and a weak field region affects the amplitude of the wave-propagated therethrough. Conventionally, the card comprises a dissipative material such as a dielectric impregnated with carbon, or of a thin layer of carbon on a dielectric sheet. In more precise attenuators, conventional materials include such materials as a thin layer of metal such as nickel-chromium alloy on a glass sheet. See U.S. Pat. No. 2,890,424 (1955) and U.S. Pat. No. 2,830,275 (1958).
Over the decades that followed, the industry saw an increase in the frequencies commonly used, however, just minor changes to the basic design were sufficient to provide adequate attenuation for these increases. In some improved conventional systems, the resistive card is made very thin, given the capability to rotate, or made of exotic materials such as tantalum nitride resistive film deposited on sapphire or alumina substrates. Although these improvements led to a more flat frequency response and improved accuracy of attenuation as systems moved to the lower millimeter wave frequencies (approximately 50 GHz), they could not adequately respond to the problems occurring as the frequencies in use moved to the millimeter to sub-millimeter range and beyond. As frequency increases, waveguide structures must necessarily shrink, and the single card insert system began losing its viability as a manufacturing product in the lowest millimeter and sub-millimeter range.
FIGS. 1 and 2 is a diagram of a conventional manual vertical insertion adjustable attenuator. Referring first to FIG. 1, resistive card 1 is attached to a micrometer assembly 3. The micrometer assembly 3 is an actuator that pushes and pulls the resistive card 1 into the path of a waveguide channel 2. At one extreme the resistive card 1 is pushed to the bottom of waveguide channel 2. Spring 4 provides tension against the downward movement of micrometer assembly 3, and is kept in place with stopper 5. Screws 6 attach resistive card 1 to a nonconductive mounting block 7, which holds and positions the resistive card 1. Spring 4, stopper 5, screws 6, nonconductive mounting block 7 and resistive card 1 all compose a resistive card holder assembly 8, which holds and positions the resistive card 1 to the center of the waveguide channel 2. In addition, a housing 9 comprises the resistive cardholder assembly 8 and the waveguide channel 2.
FIG. 2 is a diagram of the conventional manual vertical insertion adjustable attenuator shown in FIG. 1, however, FIG. 2 is shown from a perspective looking down the length of waveguide channel 2. Resistive card 1 is also shown from a side perspective. The typical thickness of a resistive card in a conventional manually adjustable attenuator is 0.005″. Importantly, resistive card 1 has a resistive surface 10. In operation, the micrometer assembly 3 pushes resistive card 1 into the path of the waveguide channel 2 through card channel 20. Card channel 20 must be wide enough to accommodate the thickness of resistive card 1, taking into account any card warping that may have occurred, and the sum of machining tolerance build-ups of all assembled parts. If card channel 20 is too narrow in a conventional system such as that shown in FIGS. 1 and 2, the card's resistive surface 10 may touch the side walls of card channel 20, effectively modifying the variable attenuation characteristics. Scratches or scrubbing on the resistive surface 10 as the resistive card 1 is traveling up and down the card channel 20 will also alter the desired attenuation response, and can even lead to binding.
The simple solution to the above problem (creating a wider card channel 20) causes excess ripples in the attenuator responses due to RF leakage into the resistive material holder assembly 8. Thus, as a current solution the industry largely handcrafts the resistive card to fit within the narrowest card channel possible without scraping or scrubbing. As typical wavelengths used continued to decreases and frequency continues to increase, the above problems are exaggerated.
At these higher frequencies, a second problem common in the conventional systems shown in FIGS. 1 and 2 relates to difficulty in precisely centering the resistive card within the waveguide channel. If the resistive card 1 is not precisely centered, the movement of the card into and out from the waveguide channel 2 causes the electromagnetic field associated therewith to be perturbed asymmetrically, since, the resistive card 1 has a dielectric medium on one side and air on the other. This leads to attenuation response variations and is amplified by the degrees of off centering as the resistive card travels down the waveguide channel. Again, the problems are made more dramatic as wavelength decreases and frequency increases.
Thus, the present application discloses an innovative manually adjustable attenuator for wave-guide use that provides improved attenuation response, particularly at the W band and above. In addition, the present invention offers improved centering of the resistive card in a waveguide channel. Finally, the present invention increases the ease of manufacturing of such adjustable attenuators.